Lightweight ceramic lens for microwave antenna

ABSTRACT

A PROCESS FOR MAKING A LENS, GENERALLY IN THE SHAPE OF A SPHERE, COMPOSED OF A DIELECTRIC MATERIAL HAVING A DIELECTRIC CONSTANT WHICH MAY HAVE A VALUE ANYWHERE BETWEEN 2 AND 20, FOR THE SPECIFIC GRAVITY RNAGE OF 0.70 TO 2.50, SUITABLE FOR USE AT A TEMPERATURE OF UP TO 1200* C., WITH CERTAIN COMPOSITIONS AND AT FREQUENCIES UP TO MICROWAVE FREQUENCIES OF 100 GHZ, COMPRISING THE STEPS OF: (1) PREPARING A POROUS, LIGHTWEIGHT, CERAMIC GRAIN; (2) BLENDING THE GRAIN WITH A MIXTURE OF ORGANIC AND CERAMIC BINDERS; (3) FORMING THE MATERIAL INTO A DESIRED SHAPE, FOLLOWED BY BURNING OUT THE ORGANIC BINDERS. THE FORMING MAY TAKE ONE OF TWO FORMS: (A) HYDROSTATIC PRESSING IN A RUBBER MOLD; OR (B) SINTERING IN A REFRACTORY MOLD, ALSO DESIGNATED KILN BONDING: (4) DRY MACHINING THE LENS TO PRECISE SPERICAL DIMENSIONS; AND (5) COATING THE MACHINED LENS WITH A CERAMIC OR CERAMICPLASTIC SEALING SYSTEM.

w. E. LENT E L 3,&2'9,403

LIGHTWEIGHT CERAMIC LENS FOR MICROWAVE ANTENNA Aug. 13, 1974 Filed July 2, 1970 CERAMIC GRAIN l6 REFRACTORY LOWER SECTION WEIGHT MOLD l2 UPPER SECTION MOVABLE FIG.

an 5300 wit mm ANGLE INVENTORS. WILLIAM E. LENT FIG.

JOSE A. FLORES BYERVIN F JOHNSTOh ATTORNEY.

JOHN-STAN, AGENT United States Patent "ice i'fiffifi material are suitable for use at very high temperatures and 3,829,403 at high microwave frequencies. More specifically, this LIGHTWE HT CERAMIC LENS FOR invention relates to a process for making a material suit- MICROWAVE ANTENNA able for fabrication of a lightweight ceramic microwave William Lent Los Angeles and Jose Flores vemce lens, generally in a spherical form, and having a diameter Califi, assignors to the United States of America as represented by the Secretary f the Navy of approximately in. This lens is intended to function Filed July 2, 1970, Ser. No. 51,875 as an antenna for missiles or aircraft, weighs approxi- C04b 21/06, 35/64 mately lb, and is of an opaque yellowish-cream color. 264-44 1 Chum Microwave lenses of the same type in the prior art 10 utilized plastic foams, dielectric-filled plastics or quartz ABSTRACT OF THE DISCLOSURE glass to form spherical lenses. The plastic foam Luneberg A process for making a lens, generally in the Shape of a lens consists of a series of concentric spherical shells. sphere composed of a dielectric material having a dielec. From the innermost shell, each consecutive shell is made tric constant which may have a value anywhere between 15 p of material having a successively lower dielectric 2 and 2 for the ifi gravity range f 7 to 250, constant, the material having the lowest dielectric constant suitable for use at a temperature of up to 1200 C., with material comprising the outer shell. The dielectric-filled certain compositions and at frequencies up to microwave plastic lenses and the quartz glass lenses have uniform frequencies of 100 gHz, comprising the steps of dielectric constants. (1) preparing a porous lightweight ceramic grain; A typical IO-inch quartz sphere used as a microwave (2) blending the grain with a mixture of organic and antenna welghed approximately 42 lbs. The focal point of ceramic binders; the quartz lens is fixed and inside the sphere, with resulting (3) forming the material into a desired shape, followed by Pattern distortion and loss of Power With an aPeTtuF e burning out the organic binders. The forming may take at the sphere surface.

one of two forms: Although the Luneberg and dielectric-filled plastic lenses (a) hydrostatic pressing in a rubber mold; or of the prior art have relatively low density, their permissintering in a refractory mold, also designated sible operative temperatures are much too low for unprokiln bonding; tected missile flight. Also, because of the stepped dielectric y machining the lens to Precise Spherical dimen' construction, the Luneberg lens is limited to lower micro- Slons; and Wave frequencies.

(5) coating the machined lens with a ceramic or ceramic- 1 ti seam te A more complete comparison of the properties of the pas c 1 gsys m.

material of this invention and that of the prior art is shown in Table 1.

TABLE I.COMPARISON OF LENS PROPERTIES Dielectric-filled quartz Ceramic lens Plastic foam Lens properties (invention) Luneberg lens Plastic lens Glass lens Dielectric constant distribution Constant Stepped Constant Constant. Upper microwave frequency limits High, to 100 gHL- Low-limited by steps. edium, up to 20 gHz. Very high. Electrical dissipation factor 0.001 .0004-.001 .0005.002 0.Q002. Power handling capability 1gh High. Side Lobes below main Lobe, db 1-622 22 18 22. Focus location. Outside or on Inside lens sphere surface. surface. (undesirable). Ability to adjust focal point. Yes Yes Yes No. Specific gravity 0.75-2.5 1.00 average 1-2 2.20. Operating temperature 1i C (phy a1 consideration only) soc-1,200 150 200 1 200. Mechanical strength forunprotectedmissile application High Verylow Low Very high.

STATEMENT OF GOVERNMENT INTEREST 5 The specific purpose of this invention is to provide ma- 5 terials and processes for fabricating a spherical ceramic The invention described herein may be manufactured antenna lens. This lens is intended to function in the nose and used by or for the Government of the United States of a missile. It is especially useful because it could possess of America for governmental purposes Without the payall the following properties: ment of any royalties thereon or therefor. (1) Operate with or without a protective radome.

(2) Withstand temperatures up to 1200 C. BACKGROUND OF THE INVENTION (3) Handle considerably more R-F power than plastic foam and plastic-filled lenses. Thrs nventionrelates to a process for malong a d1elec (4) Allow adjustment or change of dielectric constant by tric material having a dielectric constant which may have formula change.

any of a wide range of values. Certain compositions of the (5) Have excellent strength-to-weight ratio.

(6) Produce a well-defined pencil beam radiation pattern as narrow as 2.5 degrees for large diameter spheres,'with low side lobes as shown in FIG. 2.

(7) Have a density as low as 0.65 gm/cc.

(8) Exhibit low electrical losses.

(9) Be fabricated at relatively low cost.

SUMMARY OF THE INVENTION This invention relates to a process for making a dielectric material having a dielectric constant which can have any value between a great range. The material is suitable for use at a temperature which is high enough so that it may be used on high-velocity missiles. The preparation and treatment of the material involves many steps, which it is advantageous to follow in a certain order or sequence.

STATEMENT OF THE OBJECTS OF THE INVENTION It is an object of the invention to provide a method for fabricating, by molding, and high-temperature firing, a lens with uniformly distributed dielectric constant. I

It is another object of the invention to provide a method of making a ceramic lens which can focus an electromagnetic beam either outside of the sphere surface or on the sphere surface.

It is a further object of the invention to provide a material for making a lens suitable for operation on a missile, wherein high temperatures are encountered.

It is still another object of the invention to provide a material for making a ceramic lens with very low electrical dissipation.

It is yet another object of the invention to provide a material for making a ceramic lens with a very low specific gravity, lower than 0.75.

A still further object of the invention is to provide a ma terial which, depending upon the proportion of the various ingredients, has an extremely wide range in the dielectric constant, from 2 to 20, as the specific gravity is adjusted between 0.70 and 2.5.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a cross-sectional view of a refractory ceramic mold.

FIG. 2 is a transmission plot for the lightweight ceramic dielectric material of this invention, specifically of a 4- inch diameter lens.

DESCRIPTION OF THE PREFERRED EMBODIMENTS This invention relates to two different methods of producing a dielectric material having a wide range of index of refraction. The index of refraction is approximately equal to the square root of the dielectric constant. This invention resulted from an attempt to produce an improved radar lens of a spherical shape, but may be useful in other areas where a light, opaque, and refractory material having an index of refraction between 1.414 and 4.47 is required.

The main steps in the process to produce the lenses are:

(1) Preparation of a very porous lightweight ceramic gram. (2) Blending of the grain with a mixture of organic and ceramic binders. (3) Forming a lens, generally in the form of a sphere,

which is accomplished by either:

(a) hydrostatic pressing in a rubber mold, or (b) sintering in a refractory mold, also designated kiln bonding. (4) Dry machining the lens to precise spherical dimensions Coating the machined lens with a ceramic or ceramicplastic sealing system.

The hyrostatic press method requires more control of burnout of organic materials than the kiln bonding meth- 0d. However, kiln bonding requires refractory molds and more complextechniques.

Regardless of which of the two methods are used for the formation of the lens, a prefired grain must first be prepared, prior to firing the sphere. This is necessary to mini mize rupture of the sphere during organic burnout and during its firing.

The first step in the fabrication of the prefired grain is the preparation of a ceramic slip (a thick liquid).

The general formula range for the ceramic slip is shown in the second column, while the specific percentage weights for a dielectric constant of 3.253 .40 and a specific gravity of 0.700.80 is given in the third column.

TABLE 2 Weight percent bilica 0A0 10 0-15 0 l 0-10 5 Water 30-50 40 To increase operating temps., the glass frit and boron phosphate may be eliminated, the density of the resultant composition decreases, while increasing the amount of alumina. Moreover, the resultant lens is less lossy electrically. The gain of the antenna using the boron phosphate ceramic bond was 18-20 db and with the glass frit bond is 28 db.

The formula for glass frit F, premelted at 1450 C. and. water-quenched, then ball-milled to pass a 325-mesh screen, is a follows:

TABLE 3 Weight percent Calcium oxide, CaO 1-4 Magnesium oxide, MgO 1318 Alumina, A1 0 l015 Boric Acid, H BO 20-30 Silica, SiO 35-50 A proper formula selection from the ranges given in Tables 2 and 3 will make possible the required dielectric constant of 3.0 to 3.4 with a possible density range of 0.6 to 1.2 gm./cc. The dielectric constant of the material must be within the range of 3.25 to 3.40, irrespective of the composition of the material or the lens diameter for the microwave energy to be focused on or near the surface of the lens.

With an increase in the dielectric constant above the value of 3.4, the dielectric lens may focus the electromagnetic energy within the sphere, as does a quartz dielectric lens having this value. This causes power loss, optical aberration, and the R-F aperture is diflicult to locate inside the lens.

The dielectric constant, at a given density, may be adjusted by changing the ratio of titanium dioxide to silica or alumina. Specific gravity and fired hardness are decreased by lowering the percentages of boron phosphate, .glass frit, and ball clay and lower forming pressures. The composition, however, must be such that the lens have a low loss electrically. Although, for the specific application and the specific frequencies for which the material was designed it was desirable to have a dielectric constant of approximately 3.25, by the process disclosed herein a composition may be formulated having a dielectric constant of any value from 2 to 20, and even greater. As the dielectric constant becomes large, the density of the material increases drastically.

The second main step in the process to produce the ceramic lenses involves blending the following thoroughly in a paddle-type blender;

TABLE 4 Weight percent Ceramic slip'(from step 1) 38-60 Stearic acid powder, -100 mesh 30-60 Enough water to create a very thick, smooth paste.

All of the components of the ceramic slip listed in Table 2 and the components listed in Table 4 could be mixed together in one container, but it is much more difficult to thoroughly blend the lens material in this way. A Hobart mixer, ordinarily used for mixing cakes in bakeries, was used for mixing the ingredients itemized in Table 4. The mixer has a big paddle which moves in a figure-of-eight motion, and blends all the ingredients. The consistency of the resultant paste is approximately that of a thick biscuit dough.

' The second main step also involves the process of grain formation which includes the steps of:

(1) Removing water from the paste by drying on an absorbent surface or filter pressing to 6-15 moisture; (2) Extruding, while plastic, through a 28-mesh screen by any mechanical means; (3) Drying extruded, elongated, pellets until they can be reduced to approximately 28 mesh grain by a short period of slow tumbling; (4) Slowly tumbling for 3-10 minutes to approximate 28-mesh screen size grain; (5) Drying the screened grain with warm circulating air not to exceed 150 F.; and (6) Gently removing all fines smaller than 35-mesh.

The next operation consists of grain sintering, which involves:

(1) Spreading dried, sized, selected grain on a 100 mesh stainless steel screen;

(2) Holding the screen and grain, on passing them through, in a hot gas flame until the grain is charred quickly with a minimum of stearic acid melting; ignition of grain to carbonization is necessary to prevent melting and deformation before firing to maturity;

(3) After charring, sintering in a refractory sagger to 10001800 C. for one hour. The temperature choice depends upon the ceramic slip formula, which is selected for the anticipated use temperature of the lens.

The final operation in the second main step involves grain sizing, or gently breaking up the grain as required, and rescreening through a 28-mesh screen and removing all fines less than 35 mesh.

The third main step, that of fabricating a lens, and describing first the hydrostatic forming method, involves (A) Blending in a paddle type mixer at slow speed: Weight percent 2% aqueous Methocel solution 15-30 325-mesh powdered glass frit F 8-25 (B) Allowing the Methocel and glass frit to remain in mixing container, and slowly adding the prefired ceramic grain, while blending slowly so as to thoroughly coat the grain without saturating it; 75-90%;

(C) Filling the rubber mold with grain immediately after coating so as not to lose moisture;

(D) Evacuating air from the mold;

(E) Hydrostatically pressing the above evacuated mixture to 100-1000 p.s.i., depending upon desired density;

(F) Removing the mold;

(G) Burning out the organics in vented oven, from room temperature to 400 C., not to exceed 10 C./hour temperature rise; and

(H) Sintering to final temperautre, 900-1600 C., depending upon formula and desired use temperature.

Methocel is a hydroxy ethyl cellulose, manufactured by the Dow Chemical Company, Midland, Mich.

6 Lens fabrication by the kiln bonding method involves the following steps:

(A) Identical to steps A and B of hydrostatic forming method;

(B) Drying completely and rescreening to separate;

(C) Filling refractory ceramic mold 12 and 14, as shown in FIG. 1, and vibrating grains 16 into place; and after placing refractory weight 18 on mold 12;

(D) Sintering mold and grain sufficiently to join prefired grains with glass from the grain coating; and

(E) Removing sintered sphere from mold.

The fourth main step in fabricating the ceramic lens involves dry machining the sphere to precision lens dimensions.

Finally, the last main step in the fabrication involves coating the machined sphere, as follows:

(A) Base Layer:

(1) The following are ball milled for 1 hour: Weight percent Weight, percent "Potassium oxide, K 0 0-2 Magnesium oxide, M O 1-4 Calcium oxide, CaO 2-4 Alumina, A1 0 3-7 Barium oxide, BaO 6-8 Boric acid, H 1BO 16-20 Lead oxide, PbO 25-30 Silica, SiO 30-35 Coating the sphere is necessary to: (1) increase lens strength; (2) prevent surface erosion; (3) seal out moisture; and (4) provide a dielectric transition from the lens surface to air.

The process of lens formation, while it does involve some chemical reaction, such as oxidation, is primarily physical. The percentages of the various components in the final product is approximately the same as those in the original prefired material, within a few percent. Of course the organic components, including the gums and water, vanish with the firing. While the fired components are approximately the same as the unfired components, they could not be used again, even if ground up after having been fired, because they have lost their plasticity.

After exposure to heat, including the prefiring, the resultant end product, excluding the added water, weighed only 1% less due to burnout of organic material. As a result, the final product has less of a fracture problem than compositions which have a larger proportion of burn-out components.

Hydrostatic forming with a damp organic-glass-prefired gr-ain mixture is preferred over the kiln bonding technique for forming.

:In general if the prefired grain matures at temperature x C., the sphere will be fired at x-50 C., the sealing subcoat at x- C., and the sealing glass at x- C.

It can be pointed out that the mold geometry of FIG. 1 may be changed to produce a solid cylinder which may be machined into a sphere after bonding.

What is claimed is:

1. A process for making a lens composed of a dielectric material having a dielectric constant which has a value between 2 and 20 for the specific gravity range of 0.70- 2.5, suitable for use at a temperature of up to 1200' C. and at frequencies up to microwave frequencies of 100 gHz., comprising the steps of:

providing a porous, ceramic grain; blending the grain with a mixture of organic and ceramic binders;

forming the material into a desired shape; burning out the organic binders; and sintering the material into a strong ceramic; wherein the steps of providing a porous grain compromise:

milling for approximately four hours in a porcelain ball mill the following ingredients forming a ceramic slip:

Weight percent Titaniumdioxide, TiO 20-80 Silica, SiO -40 Ball clay (hydrated aluminum silicate) -15 Alumina, A1 0 O- Boron phosphate 0-15 Glass frit F 0-10 Water -50 blending the following percentages of materials thoroughly in paddle-type blender:

Weight percent Ceramic slip (from above) 38-60 Stearic acid powder 30-60 Enough water to create a thick, smooth, paste the steps of providing a porous, lightweight, ceramic grain further comprising the step of grain formation, which includes the following steps:

(1) removing water from the paste by drying on an absorbent surface or filter pressing to 615% moisture:

(2) extruding, w-hile plastic, through a 28 mesh screen by any mechanical means;

(3) drying extruded, elongated, pellets until they can be reduced to approximately 28 mesh grain by a short period of slow tumbling;

(4) slowly tumbling3-10 minutes-to approximate 28 mesh-screen size grain;

(5) drying the screened grain with; warm circulating air not to exceed 150 F.; and a (6) removing all fines smaller than '35 mesh;

the step of forming includes charring the organic binder, which further involves the following steps:

1(1) spreading dried, sized, selected grain on a 100- mesh stainless steel screen;

(2) holding the screen and grain in a gas flame until the grain is charred quickly with a minimum of stearic acid melting;

(3) after burnout, sintering in a refractory sagger to 1000-1800 for one hour, the temperature choice depending upon the ceramic slip formula and the desired lens use temperature;

(4) breaking up grain and rescreening through a 28- mesh screen, removing all fines less than mesh; 20 dry-machining the lens to precise spherical dimensions; and coating the machined lens with a ceramic sealing layer.

References Cited UNITED STATES PATENTS 25 3,366,965 1/1968 'Ochiai 343-911 3,459,503 8/1969 Roy et al. 106-39 3,534,286 10/1970 Holm et al. 264-61 3,538,205 11/1970 Gates, Jr. et al. 26461 3,288,615 11/1966 Estes et al. 26444 2,849,713 8/1958 IRobinson, Jr. 343-755 OTHER REFERENCES Ceramic Data Book, 1958-1959.

ROBERT F. WHITE, Primary 'Examiner T. P. PAVELKO, Assistant Examiner US. Cl. XJR. 

